Method of accurately measuring a value of ion beam current without interrupting the beam is disclosed in the following reference: “A cryogenic current-measuring device with nano-ampere resolution at the storage ring TARN II, Nuclear Instruments and Methods in Physics Research A 427 (1999) 455–464“(hereinafter referred to as reference 1).
This method determines a beam current value by using a superconducting quantum interference device (hereinafter referred to as a SQUID), i.e. a highly-sensitive magnetic field sensor for measuring the magnetic field generated by the beam current.
The beam current measuring device utilized in this method is primarily comprised of: (a) a detecting part operable to detect a magnetic field corresponding to beam current; (b) a magnetic flux transfer part operable to transfer magnetic flux to a measuring part; (c) the measuring part includes (i) a superconducting element being sensitive to the transferred magnetic flux and (ii) a feedback coil operable to carry feedback current for canceling out a change in the magnetic flux penetrating through the superconducting element; and (d) a magnetic shielding part comprised of a superconductor operable to magnetically shield the detecting part, the magnetic flux transfer part, and the measuring part from an external space including the space carrying ion beams.
The detecting part is comprised of a coil in which superconducting wires are wound around a core made of a soft magnetic material. The core of the soft magnetic material collects the magnetic field generated by beam current which induces superconducting current through the coil. The induced superconducting current is transferred to a coil placed adjacent to a SQUID. A change in the superconducting current flowing through the coil in response to a change in beam current attempts to change the amount of magnetic flux penetrating through the SQUID. However, the beam current measuring device is structured in such a manner that the feedback coil carries the feedback current so as to not to change the amount of magnetic flux penetrating through the SQUID, and to cancel out the change. Further, because the feedback current is proportional to the change in beam current value, measurement of the feedback current can be utilized to determine the amount of the change in the beam current value.
Recently, a method of measuring a beam current value using a high-temperature superconductor has been disclosed in “HTS Flux Concentrator For Non-Invasive Sensing Of Charged Particle Beams, ISEC 2001, page 469–470”. ISEC 2001 stands for 8th International Superconductive Electronics Conference, Jun. 19–22, 2001 Osaka, Japan.
This method uses a cylinder coated with a high-temperature superconductor on the surface thereof as a detecting section. However, on the outer peripheral surface of the cylinder, a bridge part partially made of a high-temperature superconductor is provided. Beam current penetrating through the center of the cylinder induces surface shielding current on the surface of the cylinder. The surface shielding current is concentrated on the bridge part. In this measuring method, the magnetic flux generated by the concentrated surface shielding current is measured by a SQUID.
Experiments are conducted to determine the sectional area of scattered molecular ions by placing such a beam current measuring device in a beam storage ring and measuring the attenuation of the beam current value when a circulating ion beam collides with a target gas, deviates from the orbit, and the number of ions decreases.
For example, the following demonstrates how a beam current of several hundred nanoamperes, upon colliding with the beam storage ring, attenuates to several nanoamperes for several dozen seconds.
According to reference 1, a good linearity of the output of the beam current measuring device is kept up to 2.5 μA. This is a measuring range sufficient to measure beams fluctuating from several hundred nanoamperes to several nanoamperes. Specifically, the operating point of the SQUID is locked at zero beam current, and thereafter, the attenuation of the beams colliding with the beam storage ring is measured. In other words, the amount of change from zero (operating point of the SQUID) is measured with respect to zero beam current.
However, the conventional methods are limited by a narrow measuring range. When the measuring range is widened, measuring accuracy decreases. In other words, the conventional methods cannot measure beam current of several microamperes or larger. For example, in an ion-implantation apparatus for use in semiconductor production, a semiconductor wafer is irradiated with an ion beam ranging from a microampere to several dozen milliamperes. To properly control the irradiation dose, beam current values must be measured with an error of 1% or smaller.
Application of the conventional beam current measuring device incorporating a SQUID for the use described above poses a problem. Because the linearity of the output is kept only at several microamperes or smaller, the device cannot be used. Further, when the range, in which the linearity of the output is kept is widened from a microampere to several dozen milliamperes, the sensitivity of the output with respect to the beam current has to be decreased. As a result, there is a problem of a decrease in measuring accuracy.